1. Field of the Invention
The present invention relates to an image reading apparatus which is applicable, for example, to a copying machine or a printer.
2. Descriptions of the Related Art
A recording apparatus has conventionally been known in which stripe-like image portions are successively recorded on a recording medium by performing scanning with a recording head which is equipped with a recording section having a fixed width. In a so-called digital copying machine, which has appeared on the market, a recording apparatus of the above-mentioned type is combined with an image reading apparatus which is adapted to read an original image by means of a reading device, like a CCD image sensor, and emit an electrical signal corresponding to the read image.
The assignee of the subject application has proposed in U.S. Pat. Nos. 4,876,612, 4,918,542, 4,954,913, etc. image reading systems according to which an original image is divided into a plurality of stripe-like areas which are successively read by an image sensor having a reading width smaller than the width of the original.
As shown in FIG. 2, the reading means 100 of such an image reading apparatus consists of a small-sized CCD, which is large enough to read an area having a width corresponding to the recording width of the recording head (e.g., approximately 16 mm). When reading an original image, the reading means 100 is moved, in synchronism with the printing/recording movement of the recording head, in the direction indicated by the arrow Y (the main scanning direction) while illuminating the original, thereby reading a predetermined area of the original. Then, the reading means 100 is moved in the direction indicated by the arrow X (the sub-scanning direction) so as to read the next area of the original. In this way, the entire original image is read.
FIG. 1 is a perspective view of an image reading apparatus of the type described above.
The apparatus shown includes a main scanning unit 101, which is composed of a CCD sensor, an illumination system for illuminating the original, a projection system for projecting the original image onto the CCD sensor, which sequentially converts the projected light image to electrical signals, etc. This main scanning unit 101 is supported by a pair of rails 102a and 102b in such a manner as to be movable in the main scanning direction and is moved in this direction by means of a motor 103 and a wire 104.
The main scanning unit 101, the rails 102a and 102b, the motor 103, and the wire 104 are carried by a scanning carriage 105 and constitute as a whole a sub-scanning unit 110.
The sub-scanning unit 110 is supported by a pair of rails 111a and 111b in such a manner as to be movable in the sub-scanning direction, and is intermittently moved in synchronism with the main scanning, a predetermined distance at one time, by means of wires 112a and 112b which are driven by a motor 114.
The above-described conventional image reading apparatus has the following problems: First, in moving the reading position of the CCD sensor stepwise in the sub-scanning direction to perform successive reading, the distance the sub-scanning unit is fed at one time may vary, with the result that some unevenness is generated in the boundaries between adjacent sub-scanning areas (indicated by the broken lines of FIG. 2). Such unevenness will also result from a poor adjustment of the projecting magnification portion of the optical system for projecting original images onto the CCD, i.e, the reading sensor. Thus, a very high level of precision is required of the sub-scanning feeding mechanism, and, at the same time, the optical system must be adjusted very accurately, resulting in rather high costs.
Furthermore, moving the scanning unit a plurality of times in the sub-scanning direction is likely to involve a lack of uniformity in illumination, which will cause unevenness in density between sub-scanning areas at a period corresponding to the recording width, resulting in poor image quality.